A typical permanent magnet rotor has preformed arcs of ferrite magnets (e.g., three 120.degree. arcs) glued to a solid or laminated rotor core. (Pole arcs referred to herein are in electrical degrees.) Often, a thin stainless steel cylinder or a fiber wrap over the magnets is used to restrain them against centrifugal forces and glue deterioration. More recently, rings of bonded magnets have become available; these are glued onto solid or laminated cores. When a more sinusoidal flux distribution is desired, magnets of varying thickness are glued onto the core.
Brushless dc motors are usually run in either a flat-top flux or a sinusoidal flux mode. For most applications, the flux distribution should be flat over the central 120.degree. of the pole arc. This results in relatively smooth torque output and higher torque per ampere. For position-controlled servos, for example, such as are often used in machine tools, a sinusoidal flux distribution is desirable in order to enhance the ability to hold tool position at a particular angle without excessive jitter. In motors, however, especially as the number of poles increases, it becomes difficult to maintain the flux distribution substantially flat over the full 120.degree. range.
Accordingly, it is desirable to provide a simple and economical technique for achieving a desired flux distribution in a permanent magnet rotor, particularly in rotors having molded magnets.